This invention relates to a power supply arrangement comprising a monitoring circuit for no less than two different-polarity direct voltages connected to a common reference potential, and including a signal transmission element.
Power supply arrangements comprising monitoring circuits are necessary, for example, for protecting the connected users against overvoltages in the case of failure or, in the case of undervoltages avoiding the transfer of faulty data are transferred without such transfer being timely recognised. Consequently, the failure of a direct output voltage, i.e. the deviation from a predetermined set value, is to with certainty be indicated by means of a signal transmission element so that safety measures can be taken. If a failure occurs in a secondary circuit, especially in a clocked power supply arrangement, the direct output voltages are accordingly adjusted or switched off by means of adjusting elements provided in the primary circuit. If electric isolation of the secondary circuit and the control circuit operating, for example, in the primary circuit of a transformer is necessary, a signal transmission element with electrically isolated inputs and outputs, for example, an optocoupler, is used for transmitting the control signals of the monitoring circuit.
In DE-A 37 07 973 there is disclosed a power supply arrangement comprising an overvoltage protection circuit and a signal transmission element. In this arrangement a series connection of a zener diode and a diode is inserted between a reference potential and each direct output voltage which is positive or negative with respect to this reference potential. A signal transmission element, arranged as a light-emitting diode optocoupler is inserted between the zener diode and the diode of either one of the two series connections. If no overvoltages occur, the zener diode will be blocked and the light-emitting diode will not emit. If an overvoltage occurs in a direct output voltage, the zener diode concerned will conduct and power will flow through the light-emitting diode of the signal transmission element. The accuracy of response of the signal transmission element and thus of the overvoltage protective circuit is then determined by the tolerances of a plurality of different modules, i.e. more specifically, by the tolerances of the diode, the zener diode, the signal transmission element and a dropping resistor connected in series thereto and thus has insufficient accuracy of response when more stringent requirements are involved.